1. Field of the Invention
The present invention relates to superconducting magnetometers that use the Josephson effect and are known as SQUIDs.
2. Description of the Prior Art
The use of a superconducting material to make a magnetometer using the Josephson effect is known. This device enables the detection of magnetic fields with weakness of as low as 10.sup.-15 tesla/.sqroot.Hz. The prior art in this technique is discussed in an article by Brian William Petley in La Recherche No. 133, May 1982. Until recently, superconducting materials had this type of effect only at very low temperatures, cf about a few degrees Kelvin. This meant that they had to be cooled with liquid helium. For this purpose, bulky and delicate cryostats, which are costly to operate, had to be used.
Recently, materials such as Y Ba.sub.2 Cu.sub.3 O.sub.8-x have been discovered. These materials have a superconducting critical transition temperature of about 93.degree. K., and can therefore work in liquid nitrogen which has a boiling temperature of 77.degree. K.
Gouch et al. of Birmingham University have replaced the niobium loop of a radiofrequency (RF) SQUID, available in the market, by a ring manufactured with a material of this type, and have demonstrated the existence of intrinsic loops in the ring.
Similarly, Pegrum and Donaldson of the University of Strathclyde have replaced this niobium loop by a simple piece of new material with a roughly parallelepiped shape, and have demonstrated the existence of intrinsic loops inside this material, thus opening up the possibility of detecting fields in the range of 10.sup.-8 to 10.sup.-10 T.
In these experiments, the sensitivity is limited by a noise which causes high dispersal and great uncertainty. This noise is attributed to the existence of multiple loops inside the material, leading to multiple periods which are randomly superimposed on one another. Koch et al. of the IBM Research Center, Yorktown Heights, have built a SQUID in which the sensitive element used is a thin film of this new material, machined by ion implantation to form an adequate loop. Unlike the one used in the above experiments, this SQUID is of the DC type and therefore works with a DC current bias. However, it has not been possible to obtain superconductive operation at temperatures above 68.degree. K., and the results obtained clearly show that the loop used is superconductive only on a small portion and that the curves are, in fact, akin to a network of junctions with a poorly defined transition wherein the effect observed corresponds to a resistive commutation assisted by the Joule effect of the normal neighboring conductor.